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94

SODIUM BISULPHITE

grams of pure sodium chloride are then added and the whole cooled to o
and titrated with N-hydrochoric acid in presence of plicnolphthalein until
the red coloration disappears (I c.c. N-acid = 0-053 gram NaaCO3). Methyl
orange is next added and the titration continued with the same acid to a
red colour (i c.c, N-acid = 0-084 Sram NaHC03).

I'

SODIUM BISULPHATE

NaHS04+HaO=i38

Colourless crystals or white, fused masses, soluble in water to an acid
solution. It may contain the same impurities as the sulphate and these
are detected by similar methods (sw Sodium Sulphate). The content of
pure sodium bisulphate is determined thus :

Quantitative Determination.- -4--5 grams are dissolved in water and
titrated with normal alkali (methyl orange) : I c.c. N-alkali  0-120 grain
NaHS04 or 0-0138 gram NaliSO, + H,().

SODIUM   BISULPHITE
NallSO.,      104-1  (104)

This is put on the market in crystals, or moist, crystalline masses, or dry
white powder, or solution ; the crystals and the dry powder are usually
odourless, but. the other qualities smell of sulphur dioxide. It is soluble
in water giving an acid solution.

It may contain the same impurities as the neutral sulphite and these
are similarly detected (see* Sodium Sulphite) ; iron and sulphate especially
should be tested for. Its value depends essentially on the content of bisul-
phite or sulphur dioxide (free ; semi-combined, that is, as bisulphite;; com-
bined, that is, as normal sulphite), which is determined as follows :

Quantitative Determination.- -This is basal on the facts that the
total, sulphur dioxide may be estimated by titration with iodine and that
the free or semi-free sulphur dioxide reads acid towards plienolphthalein,
whereas the semi-free has a neutral reaction towards mctbyl orange. In
other words : with phenolphtihalein only the .normal sulphite reacts neutral,
while with methyl orange the bisulphite also reacts neutral ; free sulphur
dioxide is acid towards both indicators.

(a) TOTAL SULPHUR DIOXIDE. -5 grams are dissolved to x litre in
recently boiled and cooled water, this solution being run from a burette into
15 c.c. N/io-iodine solution in a flask until the liquid is almost deeolo.lsed ;
starch paste, is then added and the titration continued until the blue, colour
disappears: i c.c. N/io-iodine -- 0-0032 gram S()2.

(6) FREE AND SEMI-FREE SULPHUR DIOXIDE, -200 c.c. of the solution
prepared as in (a) are titrated with N-sodinm hydroxide in presence of
methyl orange until a yellow coloration is attained ; this gives the free
SOa (i c.c. N-alkali -- 0-064 gram SOS). Phenolphthalein is then added
and the N-sodiurn hydroxide run in until a red colour appears ; this gives
the semi-free S0a (i c.c. N-alkali  - 0-032 gram S08). These two valuesthan 2% of normal carbonate).    Quantitative